Extrude Arch like in the video
No Addon, plain Blender.

The arch was mirrored.

The mirrored arch is extruded along the normals and meet in the middle. Like the extrusion in the video.

Here the view with only one side editable:


Side note:
Without the mirror if you have the whole arch. You can archive something similar with extrude followed by scale. e then s